Experimental Design

I. Definition of Experimental Design

II. Simple Experimental Design

III. Complex Experimental Design

IV. Quasi-Experimental Design

V. Threats to Validity

Experimental Design - Definition

Control over the sequence and proportion of the independent variable involving: 1) at least two conditions (i.e. an independent variable); 2) random assignment of subjects to conditions; and 3) the measurement of some outcome (i.e. dependent variable

Experimental Design – Simple Designs

2. Pre-Post-test Control Group Designs (t-test)

E R O1 X -> O3

C R O2 -> O4

1. Post-test Control Group Designs (t-test)

E R X -> O1

C R -> O2

Experimental Design – Simple Designs (cont.)

3. Soloman Four Group Design (t-test)

E1 R X1 -> O1

C1 R -> O2

E2 R O1 X1 -> O3

C2 R O2 -> O4

4. Analysis of Variance (ANOVA)

E1 R X1 -> O1

E2 R X2 -> O2

E3 R X3 -> O3

Experimental Design – Complex Designs

Main Effects

Interaction Effects

1. Completely Randomized Designs (CRD) (This example is a 2x3 CRD)

  C-E1 C-E2 C-E3

R-E1 O11 O12 O13

R-E2 O21 O22 O23

Experimental Design Complex Designs (cont.)

2. Incomplete Designs (IRD)

Split Plot Design (This example is a 2x3 SPD)

C-E1 C-E2 C-E3

R-E1 - O12 O13

R-E2 O21 O22 -

Experimental Design Complex Designs (cont.)

3. Repeated Measures Designs (RMD)

Latin Square Design ( This example is a 4x4 LSD)

O1 O2 O3 O4 O2 O3 O4 O1 O3 O4 O1 O2

O4 O1 O2 O3

Experimental Design – Quasi Experiments

1. One Shot Case Study

E O1 X ->O2

2. Non-Equivalent Control Group Design

E O1 X -> O3

C O2 -> O4

3. Interrupted Time-Series Design

E O1 O2 O3 X O4 O5 O6

Experimental Design - Threats to Validity

1. History = confounding of IV over time

2. Maturation = age / experience contaminate

3. Testing = subjects come to understanding IV

4. Regression to the Mean = extreme scores regress

5. Selection of Participants = non-random assignment

6. Mortality = subject attrition

7. Diffusion of Treatments = lack of control group